I've been integrating Infopath as a solution for many of our customers, and quite frankly, it takes a fair deal of pestering for it to play ball. The COM objects are hardly exposed to the outside world, making is near impossible for seamless communication between our client application and Infopath sitting on the same machine.
Infopath templates ( that define the look and structure of the form ) have a suffix called .XSN, which in fact is just a .CAB file with a different suffix. Inside this CAB are various files including Xsd files (the form's structure) and VBScript or JScript script files (the form's validation, logic and whatever else you may want it to do).
Such is the direction of Microsoft, that it is now possible to use managed code as well. I have yet to investigate the full implications of this, but I will be testing them out in my forthcoming integration project. The fully usable preview Infopath SP1 can be found here :
http://www.microsoft.com/downloads/details.aspx?familyid=D5ADC839-73F4-4299-ABA0-E88C90B25144&displaylang=en